Rigging the roulette wheel to slow the spread of viruses

By Rhea Hirshman

Avian flu virus is not new. All the influenza strains that affect people have avian origins—including the virus that caused the 1918 - 1919 influenza pandemic, according to Durland Fish, Ph.D., professor of epidemiology. That outbreak killed at least half a million people in the United States and more than 30 million worldwide. As with such other “emerging” diseases as Ebola hemorrhagic fever, Lyme disease and severe acute respiratory syndrome, or SARS, what is “new” about avian flu is a heightened awareness of the role of other species in the development of this human health hazard, says Fish. That makes it a perfect case study for the new center Fish directs, the Center for EcoEpidemiology, part of the Yale Institute for Biospheric Studies. The center’s purpose is to bring together experts in ecology and epidemiology in areas where their studies intersect.

Fish highlighted one such intersection in a talk at a conference on campus last May, “Ethical Aspects of Avian Influenza Pandemic Preparedness, Part 1: Vaccines,” when he discussed the necessity of focusing more resources on understanding how the virus evolves and functions in wildlife populations so that we can keep it from developing into a strain transmissible to humans.

Of the numerous strains of the influenza virus, of most concern is the H5N1 strain. “A common scenario is that avian viruses in wild-bird populations are transmitted to domestic birds or sometimes to pigs. New strains can evolve when animals are in proximity to each other,” Fish says. This process is known as “recombining.” Although humans have developed some immunity to various influenza genotypes through exposure, they would be highly susceptible to the new strain were it to become widespread in the human population.

What could make that happen? Fish says that we do not know why some strains jump the species barrier to humans and others do not. “We do know,” he says, “that the H5N1 strain recombined in domestic animals and is now back in the wildlife population. We saw the first human cases in the late 1990s. When several people in Southeast Asia died of a previously unknown influenza virus, the virus was studied and we now know it as H5N1.”

A pandemic occurs when a virus introduced into the human population through another species moves from person to person. Rather than relying on the wholesale destruction of infected and susceptible bird populations, or pouring resources into developing a human vaccine (he notes the impracticality of vaccinating billions of people quickly), Fish suggests other ways to keep a pandemic at bay.

“The first,” he says, “is to vaccinate carrier species against viral genotypes with pandemic potential by developing oral vaccines that can be distributed to wild birds through feed.” Another approach would involve the introduction of a variation of the virus that produces a milder form of the infection but still maintains itself in appropriate bird populations, thus rendering the birds immune to pandemic genotypes.

Fish would like to see greater cooperation between the disciplines of ecology and epidemiology. “We should preserve wildlife, while at the same time figuring out how to keep it from being a threat. If we can buy time and learn more about the evolution of viruses in nature, we can rig the roulette wheel rather than just waiting for it to turn.”